Database Error

I’m not sure if this is a problem with WordPress or the theme I’m using, but all of a sudden after months of this website being fine, I get this: Error Establishing a database connection. The wp-config file hasn’t changed, and the control panel of the website says the database is fine.

I am willing to pass on the admin details of the site if someone can genuinely find out of this is the recent version of wordpress doing this or what?